1. Field of the Invention
The invention relates to an annular combustion chamber for a gas turbine in which a combustion-chamber dome is arranged upstream of an air-cooled combustion chamber, at least a portion of an air flow which comes from the compressor being directed via cooling ducts which run at least in sections along the combustion chamber, the cooling ducts having an entry into the combustion-chamber dome.
2. Discussion of Background
In conventional combustion chambers, a first portion of an air flow coming from the compressor is fed as combustion air to the burners seated in the dome. A second (in general the larger) portion of the air flow coming from the compressor is directed through cooling ducts, which run at least in sections along the inner and/or outer combustion chamber, and convectively cools the combustion-chamber walls in order to then likewise be collected in the combustion-chamber dome and fed to the combustion operation. A small portion of the cooling air directed through the cooling ducts is possibly fed into the combustion chamber in order to cool the combustion-chamber walls.
In combustion chambers as used in particular in gas turbines, undesirable thermoacoustic oscillations often occur. These thermoacoustic oscillations involve a mutually amplifying interaction of thermal and acoustic disturbances. In the process, undesirably high oscillation amplitudes may occur and these may lead to high mechanical loading of the combustion chamber, an increase in the emissions due to inhomongeneous combustion, and in the extreme case to extinction of the flame. In conventional combustion chambers, the cooling by means of blowing cooling air into the combustion chamber along the combustion-chamber walls generally plays an important role in the sound damping and thus in the damping of such oscillations of the combustion chamber. In order to achieve low NO.sub.X emissions, an increasing proportion of the air is directed through the burner itself in modern gas turbines; the blowing-in of the cooling air is therefore reduced. In order to reduce the thermal loading of the combustion-chamber walls to a tolerable level, the combustion-chamber walls are cooled convectively. The air required for this is collected in the combustion-chamber dome and thus fed to the burners. Due to the lower degree of sound damping accompanied by this, the problems referred to at the beginning and associated with the undesirable oscillations consequently occur to an increased extent in such modern combustion chambers.
Experience has now shown that thermoacoustic oscillations or instability occur in particular close to the natural oscillations of the oscillating system. In this case, the oscillating system consists of the combustion chamber and also, depending on the acoustic boundary conditions, adjoining volumes, for instance the combustion-chamber dome. The frequencies of the pressure and velocity fluctuations then lie close to the natural frequencies of the system, and also the oscillation modes, that is, the spatial structure of the oscillation amplitudes correspond approximately to one of the natural modes of the system.
In modern gas turbines, annular combustion chambers of small radial extent are often used. When discussing oscillations and oscillation modes in such annular combustion chambers, it is advisable to use cylindrical coordinates .rho., .phi., z. The latter are related to the right-angled Cartesian coordinates x, y, z in a conventional manner via x=.rho. cos y, .rho.= sin .phi., z=z. The radial coordinate .rho. is essentially constant in the case of an annular combustion chamber of small axial extent and is equal to the radius of the combustion chamber R. The coordinate z runs along the axis of the combustion chamber. The angle .phi. runs around the axis of the combustion chamber. It is designated below as azimuth. Oscillation modes in which the pressure fluctuations exhibit no variation in the z-direction but only a variation with the azimuth angle .phi. are called purely azimuthal oscillation modes.
In the said annular combustion chambers of small axial extent, experience shows that the purely azimuthal modes, in particular the azimuthal fundamental mode, are especially important. Since the combustion-chamber radius in such combustion chambers is typically as large as the length of the combustion chamber and thus the circumference of the combustion chamber is substantially larger than the length, these azimuthal modes also have the lowest oscillation frequencies. In this case, the oscillation frequency of the purely azimuthal modes is an integral multiple of the azimuthal fundamental mode. Experience shows that the damping of the lowest thermoacoustic oscillations is especially important.
Purely azimuthal oscillations are only possible if both ends of the oscillating system have a high acoustic impedance. This condition is certainly fulfilled at one end--the combustion-chamber discharge to the turbine. Since the pressure drop at the burners is relatively small, the combustion-chamber dome must be included in the consideration at the other end.
There, the combustion-chamber dome then forms the acoustically hard end (high impedance) of the oscillating system. The acoustic impedance of the combustion-chamber dome can now be reduced by enlarging the bypass openings. However, this is not a practical way in many cases, since, for example, the cooling-air mass flows are further reduced as a result.
It is also possible to use Helmholz resonators, as described, for instance, in EP-A1 0 597 138, in the region of the dome. However, this is not always possible, for reasons of space. Helmholz resonators also act only within a narrow frequency band around a fundamental frequency. If a plurality of modes having various frequencies are to be damped, a series of Helmholz resonators would be necessary, which requires a large amount of space and considerable design input.